1. Field of the Invention
The present invention relates to an infrared detector using a photoconductive infrared detector, and also to a gas analyzer having the configuration of such an infrared detector.
2. Related Art
As a device which measures the concentration of carbon dioxide in a respiratory gas, known is a non-dispersive infrared analyzer. A carbon dioxide analyzer using the principle of the device is disclosed in U.S. Pat. No. 5,153,436.
The non-dispersive infrared analyzer measures a gas concentration such that an infrared radiation through the gas to measure an amount of attenuation of the infrared radiation in wavelength absorbed by the gas. In U.S. Pat. No. 5,153,436, in order to increase the measurement accuracy, there is employed two infrared radiation detectors. The light passing through an airway adapter for guiding a respiratory gas is divided in two lights by a beam splitter. A light reflected from the bean splitter is guided to a first PbSe detector via an optical filter. This filter allows to pass a light having a band of approximately 4.3 .mu.m which is absorbed by carbon oxide gas.
A light passing through the beam splitter is guided to a second PbSe detector via an optical filter different from the optical filter located in the first PbSe detector. This filter allows to pass a light having a band of approximately 3.7 .mu.m which is not absorbed by carbon dioxide gas.
Assuming that output signal Vs represents the amount of the infrared light guided into the first PbSe detector and output signal Vr represents the amount of the infrared light guided in to the second PbSe detector, the gas concentration can be detected without being affected by a drift due to variation of infrared radiation of the light source, by calculating a ratio of VS/VR.
Even when the ratio of VS/VR is obtained in this way, the temperature drift remains to affect the detection because the first and second PbSe detectors are different from each other in temperature coefficient of the sensitivity. In the conventional art, therefore, the temperatures of PbSe detectors are controlled by using heaters and thermistors.
In the conventional art, as described above, heaters, thermistors, and a temperature control circuit are necessary in order to maintain a photoconductive infrared detector at a constant temperature. Therefore, there arise problems in that the production cost is high, that the power consumption is increased, and that the warm-up period is prolonged.